Personnel at the U.S. military's headquarters were stunned by the first terrorist assault on the building in its 58 years.

WASHINGTON -- A hijacked passenger plane sliced into the Pentagon on Tuesday, triggering a thunderous explosion and fierce fires in the defense complex and killing and wounding an unknown number of people.

[AP photos]Rescue workers look at a wrecked section of the Pentagon, which was hit by a hijacked passenger jet Tuesday morning about an hour after the attacks in New York City. More than 10 hours later, Secretary of Defense Donald Rumsfeld held a press briefing from the building, saying "The Pentagon is functioning. It will be in business tomorrow."

The surprise terrorist assault, the first in the history of the 58-year-old building, came within an hour of the attack on the Twin Towers of the World Trade Center and set off a state of emergency in the nation's capital that swiftly shut down the government. All federal office buildings were closed and F-16 fighter jets and helicopters scrambled to police the skies.

The choking, acrid smell of smoke engulfing the area and the sight of men and women trained for war fleeing in shock and fear underscored the vulnerability of the American military and the inability of the most sophisticated early warning systems in the world to stop a low-tech form of terrorism: hijacking.

American Airlines Flight 77, a Boeing 757 carrying 64 passengers and six crew members, was on a scheduled flight from Dulles International Airport west of Washington to Los Angeles when it flew low and slammed into the five-sided, five-story concrete-walled structure about 9:30 a.m., when Pentagon workers are already deep into their workday.

More than 10 hours after the terrorist attack, Secretary of Defense Donald Rumsfeld struggled to give the impression of business-as-usual in a brief appearance in the Pentagon press room. Noting that the briefing was taking place in the Pentagon, Rumsfeld announced, "The Pentagon is functioning. It will be in business tomorrow."

A Pentagon employee receives aid Tuesday morning after the U.S. military headquarters was hit by a hijacked passenger jet. Officials Tuesday could not estimate the number of casualties in the assault.

But he declined to answer a question about whether the Pentagon had any advance knowledge that such an attack was planned, saying that he did not discuss intelligence matters, and said he could give no reliable estimate of casualties.

Rumsfeld was joined by Gen. Henry Shelton, chairman of the Joint Chiefs of Staff, who returned Tuesday afternoon to Washington from Europe. Shelton condemned what he called an "outrageous act of barbaric terrorism carried out by fanatics against both civilians and military people, acts that have killed and maimed many innocent and decent citizens of our country." He added, "Make no mistake about it. Your armed forces are ready."

Many of the Pentagon's more than 20,000 civilians and military men and women were already on edge when the attack came. News of the crashes at the World Trade Center had shot through the corridors and it seemed as if every office television was turned on. Military and civilian employees watched in astonishment and horror as smoke engulfed the two towers and shock enveloped New York City.

In a macabre foreshadowing of what then happened, Mike Slater, a former Marine, told his fellow workers, "We're next."

Then the building, built to withstand terrorist attacks, shook like a rickety roller coaster. A section of it collapsed and burned. "It sounded like a roar," said Slater, who was 500 yards from where the jet slammed into the Pentagon's west side.

Slater said he braced himself for a second explosion since he knew there had been two airplanes that crashed into the Twin Towers in New York. Instead, blue-and-white strobe lights and wailing sirens alerted those inside to evacuate. Evacuation orders were also sounded over a loudspeaker. Smoke quickly filled the air, but the lights stayed on.

Indeed, shortly after the evacuation, warnings were broadcast of a reported second plane approaching the building, but it didn't come.

As soon as Slater stepped outside, he saw and smelled something uncomfortably familiar. "I saw a mass of oily smoke and thought of the oil fields of Kuwait," he said. "There were 3,000 Americans killed in Pearl Harbor. This will be at least that many, if not more, and I hope Congress has the guts to do something about it."

When the Pentagon was built as a fireproof, air-conditioned headquarters for the U.S. military in a record 16 months in the 1940s, it was touted as an engineering marvel. Even now, the five-sided, five-story building, which has three times the floor space of the Empire State Building and houses 24,000 employees, is called one of the architectural achievements of the 20th century.

Over the years, there have been a number of terrorist bomb threats that resulted in tightened security at the Pentagon. In 1987, a 29-year-old gunman was shot and killed at one of the Pentagon's entrances after he tried to enter an area near the National Military Command Center. But never before had there been a terrorist attack against the Pentagon.

Rumsfeld was in his office on the third floor of the outer ring when he heard and felt the crash on the other side of the building.